1. Field of the Invention
The present invention relates to a method of supervising the filling level in hoppers, shafts, silos and the like for miscellaneous materials, such as blaststone, broken and crushed mineral products.
2. The Prior Art
Previously a plummet has often been used to supervise the filling level in hoppers, this being lowered to the surface of the material which has been poured in and the length of the plummet line then being measured or recorded. In general this method can only be used in vertical hoppers or the like, and it also means that the flow of material must be discontinued before the plummet is lowered, which causes practical drawbacks.
Another method used for the same purpose is that of emitting an acoustic signal towards said surface, receiving the reflected echo and, from the transmission time of the sound, determining the level of said surface. However, the use of this method is limited to hoppers of about 30 m deep and furthermore they must be at least substantially straight if the method is to be reliable. Furthermore, the use of this method is complicated in the case of magnetic materials such as iron ore, because the emitters used are provided with permanent magnets and these become coated with magnetic dust which disturbs the measuring function in an unacceptable manner.
Other methods using conducting or capacitive sensing members which are lowered into the hopper to be influenced by the material cannot be used for chunky objects which are often thrown down from a considerable height, as the sensing members would very quickly be destroyed. It has therefore been impossible to use these methods in practice.
Another method, which however has its limitations, is to locate radioactive elements at various levels down one side of the hopper, the radiation from these elements being recorded by a sensing member on the opposite side of the hopper. However, it has not been found practical to use this method for chunky objects in view of the risk of damage to both the radioactive elements and the sensing members, with consequent risk to personnel. Furthermore, it is often difficult to gain access to the various levels in the hopper to assemble the equipment.
Recently a method has been used, particularly in mining shafts, in which the weight and/or volume of material fed in is recorded, from which the filling level in the hopper can be calculated. However, this method may give considerable measuring errors which are gradually accumulated.